Matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS) is typically performed by depositing a sample containing analyte on a solid support in the presence of a photon-absorbing matrix (usually a crystalline organic acid) (Karas, et al., Anal. Chem., 60:2299-2301 (1988)). When exposed to pulses of laser light, the matrix transfers energy to molecules in the sample, thereby promoting their ionization and desorption. Because this technique is relatively gentle, large biological molecules such as proteins can be volatized without degradation or fragmentation (Karas et al. (1987) Int. J. Mass. Spectrom. Ion Processes 78:53 (1987); Spengler, et al., Analysis 20:91(1992)). Once in the gas phase, the ions are transferred into a mass spectrometer and accelerated toward a detector by exposing them to an electric and/or a magnetic field. Time of flight (TOF), Ion Cyclotron Resonance (ICR), linear trap quadrupole, orbitrap based analysis can be used to determine the mass-to charge ratio of the molecules volatized from the sample.
Recently, attempts have been made to apply MALDI MS techniques diagnostically by identifying spectral profiles characteristic of disease states (Stoeckli, et al., Nat. Med. 7:493 (2001); Rahman et al., Am. J. Respir. Crit. Care Med. 172:1556 (2005); Yanagisawa, et al., Lancet 362:433 (2003); Chaurand, et al., Curr. Opin. Biotechnol. 17:431(2006)). Using samples of cells and tissues, hundreds of peptide and protein peaks can be recorded in a mass spectrum produced from a single sample or from a single site on a sample (WO 2007/139989, Agar, et al., Anal. Chem. 79:7416-7423 (2007)). Moreover, scans of tissues may be performed in which hundreds of closely spaced MALDI MS spectra are taken in a grid pattern covering the entire sample (Gusev, et al. Anal. Chem. 67:4565 (1995); Stoeckli, et al., J. Am. Soc. Mass Spectrom. 10:67 (1999); Caprioli, et al., Anal. Chem. 69:4751 (1997)). Overall, MALDI Mass Spectrometry Imaging (MSI) has been shown to have the potential to rapidly detect diseased cells and reveal how they are distributed in tissues. This may be of enormous value in, for example, in making treatment decisions dependent upon the identity and aggressiveness of tumors, and the ability to predict a response to treatment.
Despite its promise, MS imaging has not yet been used clinically as a diagnostic tool to a significant degree. In part, this is because inconsistencies in the size and distribution of matrix particles on support surfaces creates site to site variability in the amount of energy transferred and, consequently, in molecule desorption. As a result, spectra do not accurately reflect the distribution of molecules in tissues and are only poorly reproduced from one analysis to the next. Thus, better methods for preparing matrix surfaces for MALDI MS are critical to the advance of this technology.